WA has started to use chemicals about five years ago to help keep roads from getting slick. Since we have so much rain it helps to clean any of the chemicals off before it has a chance to do much damage.
As far as mountain equipped buses are concerned, few WA buses were equipped with auxiliary braking systems. But with the advent of the ISB and the fact exhaust braking is only a $100.00 option more and more of the newer buses are coming equipped with exhaust braking.
As far as big HP is concerned, the gradability spe'c that WA requires buses to meet pretty much requires the biggest HP options on the biggest buses.
Here is a link to some light reading on what is required on any school bus made since 1977, as revised in 2015, as far as HP and torque is concerned. You may have to cut and past the address rather than clicking on the link.